1. Field of the Invention
This invention relates to differential thermal analyzers, such as differential scanning calorimeters, which can be heated and cooled very rapidly.
2. Background of the Invention
Differential thermal analyzers (DTA) measure the difference in temperature between a sample material and a reference material as the sample and reference materials are simultaneously subjected to dynamically controlled changes of temperature. Measurement of the dynamic temperature difference as a function of the sample temperature or of time gives qualitative and quantitative information concerning physical transformations which occur in the sample material. Differential scanning calorimeters (DSCs) are differential thermal analyzers wherein the heat flow to and from the sample material is measured quantitatively.
The heating and cooling rates which can be obtained, and the ability to rapidly equilibrate at a desired temperature are important performance characteristics for differential thermal analyzers. For example, "Isothermal Crystallization," is a measurement of the elapsed time for crystallization of a sample. The experiment consists essentially of heating a material to a temperature above its melting point, and holding it at that temperature until all crystals in the material have melted. The temperature of the sample is then reduced rapidly to a predetermined temperature below the melting point of the crystal and held at that temperature as the material solidifies and crystals grow. The record of differential temperature versus time will show an exothermic peak. That peak records crystallization of the material. The time at which the maximum temperature difference occurs is taken as the crystallization time.
In this measurement, the instrument must reduce the sample temperature from above its melting point to the isothermal temperature as rapidly as possible, and must stabilize the sample temperature at the isothermal temperature very quickly without allowing the sample to cool significantly below the isothermal temperature. Typical specifications for a differential thermal analyzer for isothermal crystallization measurements includes cooling the sample at 200.degree. C./min. and stabilizing the sample temperature at an isothermal temperature in 30 seconds, without undershooting the isothermal temperature by more than 0.5.degree. C.
Differential thermal analyzers include the following major components: (1) holders for the sample and the reference materials, (2) a sensor to measure the temperature difference between the sample and the reference, (3) a sensor to measure the temperature of the sample, and (4) an oven to heat the sample and reference materials.
Most typically, the oven consists of a high conductivity metal block (usually silver) wound with a resistance heating element enclosed in a thermally insulating housing. The oven may also be equipped with a cooling system to remove heat from the oven. The large mass of the oven usually limits the cooling rates to well below the minimum required specification for isothermal crystallization because the cooling system must cool the relatively massive furnace in order to cool the sample. By using cryogenic liquids or multistage mechanical refrigeration cooling systems, conventional differential thermal analyzers have cooling rates up to 50.degree. C./min., over a limited range of temperatures. They usually cannot achieve isothermal temperature stability within the desired time, and with the allowable temperature undershoot. Thus conventional DTA instruments cannot be used satisfactorily for isothermal crystallization measurements.
High density infrared heating uses radiation emitted by infrared (IR) heat lamps to heat the surface of an object. Typically, tubular IR heat lamps are used with either elliptical or parabolic reflectors, which direct and focus the radiation onto the object. The reflectors are usually metallic with a reflective coating having very high specular reflectance in the IR region of the electromagnetic spectrum (i.e., wavelengths between 1 .mu.m and 1 mm). Gold or silver coatings are very effective IR reflective coatings, although gold coatings are generally preferred.
In elliptical reflectors, the IR lamp is positioned at one focus of the ellipse, and the radiant energy emitted by the lamp is focussed by the reflector onto a line located at the opposite focus of the ellipse. In this manner very nearly all of the IR energy emitted by the lamp is concentrated along this focal line, resulting in very high energy densities. By arranging multiple reflectors so that the heated focus of each reflector is collinear, the energy from multiple IR lamps may be focussed along the same line, increasing the energy delivered to the heated focus in proportion to the number of IR lamps and reflectors used.
Parabolic reflectors are used with the IR lamp positioned at the focus of the reflector so that the emitted radiant energy is reflected in parallel rays. Thus, IR heaters employing parabolic reflectors do not deliver the same high energy densities as those having elliptical reflectors, but are well-suited for heating plane surfaces. Multiple parabolic reflector IR heaters may be arranged so that the parallel rays emitted by each assembly intersect, creating a heated region having a large volume. Alternatively, multiple parabolic reflector IR heaters may be arranged to radiate on a surface, thus increasing the energy density at the heated surface.
Because of the very high energy densities attained using IR lamps, very high heating rates can be achieved. Depending on the characteristics of the heated load, especially the load mass, heating rates as high as several thousand .degree. C. per minute have been achieved.
High density IR heating has been used in thermal analysis instruments, for example, in thermogravimetric analyzers (TGA), differential thermal analyzers (DTA), differential scanning calorimeters (DSC), combined TGA and DTA, and combined TGA and DSC. However, none of these systems have combined an active cooling mechanism with IR heating to achieve the high heating rates, rapid cooling rates and precise temperature control of the present invention.